Hybrid Vapor Phase-Solution Phase Growth Techniques for Improved CZT(S,Se) Photovoltaic Device Performance

ABSTRACT

A hybrid vapor phase-solution phase CZT(S,Se) growth technique is provided. In one aspect, a method of forming a kesterite absorber material on a substrate includes the steps of: depositing a layer of a first kesterite material on the substrate using a vapor phase deposition process, wherein the first kesterite material includes Cu, Zn, Sn, and at least one of S and Se; annealing the first kesterite material to crystallize the first kesterite material; and depositing a layer of a second kesterite material on a side of the first kesterite material opposite the substrate using a solution phase deposition process, wherein the second kesterite material includes Cu, Zn, Sn, and at least one of S and Se, wherein the first kesterite material and the second kesterite material form a multi-layer stack of the absorber material on the substrate. A photovoltaic device and method of formation thereof are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/342,618 filed on Nov. 3, 2016 which is a divisional of U.S.application Ser. No. 14/540,986 filed on Nov. 13, 2014, now U.S. Pat.No. 9,530,908, the contents of each of which are incorporated byreference herein.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with Government support under Contract numberDE-EE-0006334 awarded by Department of Energy. The Government hascertain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to CZT(S,Se)-based photovoltaic devicesand more particularly, to a hybrid vapor phase-solution phase CZT(S,Se)growth technique.

BACKGROUND OF THE INVENTION

CZT(S,Se) is an earth abundant, light absorbing material presently beingused in photovoltaic devices. The highest efficiency photovoltaicdevices have been achieved via growth of the absorber by solutiondeposition with the elemental constituents being dissolved in hydrazine.See, for example, U.S. Patent Application Publication Number2013/0037090 filed by Bag et al., entitled “Capping Layers for ImprovedCrystallization.” CZT(S,Se)-based photovoltaic devices have beenproduced with efficiencies above 12% but these devices unfortunatelysuffer from point defects (crystallographic defects) which limitincreases in open circuit voltage (Voc).

One approach to increasing Voc involves the use of high work functionback contact materials to replace the standard molybdenum (Mo) metalcontact. See, for example, U.S. Patent Application Publication Number2013/0269764 filed by Barkhouse et al., entitled “Back Contact WorkFunction Modification for Increasing CZTSSe Thin Film PhotovoltaicEfficiency” (hereinafter “U.S. Patent Application Publication Number2013/0269764”). However, while the standard device thickness isgenerally about 2 micrometers (μm), contact engineering to achieveincreases in Voc works only if the thickness of the CZTSSe absorber isreduced to below 1 ptm. This critical dimension is determined by the sumof the back and front depletion widths and minority carrier diffusionlength. See, for example, U.S. Patent Application Publication Number2013/0269764. Above this value the device will not show improvement inVoc.

While sub-micron films can be fabricated with solution phase techniques,the resulting films suffer from pinholes. Pinholes are undesirable sincethey can lead to electrical shorting of the device when conductivecontacts are added. Additionally, electron blocking layers or holetransport layers (often oxides) have been adopted in thin film solarcells for improved rectifying behavior and Voc. Oxides, however, getreduced by the hydrazine used in solution phase deposition, which isundesirable.

Therefore, techniques for forming high efficiency, defect-freeCZTS(S,Se) absorber photovoltaic devices would be desirable.

SUMMARY OF THE INVENTION

The present invention provides a hybrid vapor phase-solution phaseCZT(S,Se) growth technique. In one aspect of the invention, a method offorming a kesterite absorber material on a substrate is provided. Themethod includes the steps of: depositing a layer of a first kesteritematerial on the substrate using a vapor phase deposition process,wherein the first kesterite material includes copper (Cu), zinc (Zn),tin (Sn), and at least one of sulfur (S) and selenium (Se); annealingthe first kesterite material to crystallize the first kesteritematerial; and depositing a layer of a second kesterite material on aside of the first kesterite material opposite the substrate using asolution phase deposition process, wherein the second kesterite materialincludes Cu, Zn, Sn, and at least one of S and Se, wherein the firstkesterite material and the second kesterite material form a multi-layerstack of the absorber material on the substrate.

In another aspect of the invention, a method of forming a photovoltaicdevice is provided. The method includes the steps of: depositing a layerof a first kesterite material on a substrate using a vapor phasedeposition process, wherein the first kesterite material includes Cu,Zn, Sn, and at least one of S and Se; annealing the first kesteritematerial to crystallize the first kesterite material; depositing a layerof a second kesterite material on a side of the first kesterite materialopposite the substrate using a solution phase deposition process,wherein the second kesterite material includes Cu, Zn, Sn, and at leastone of S and Se, and wherein the first kesterite material and the secondkesterite material form a multi-layer stack of an absorber material onthe substrate; forming a buffer layer on a side of the multi-layer stackof the absorber material opposite the substrate; and forming atransparent front contact on a side of the buffer layer opposite themulti-layer stack of the absorber material.

In yet another aspect of the invention, a photovoltaic device isprovided. The photovoltaic device includes a substrate; a layer of afirst kesterite material on the substrate, wherein the first kesteritematerial includes Cu, Zn, Sn, and at least one of S and Se; a layer of asecond kesterite material on a side of the first kesterite materialopposite the substrate, wherein the second kesterite material includesCu, Zn, Sn, and at least one of S and Se, and wherein the firstkesterite material and the second kesterite material form a multi-layerstack of an absorber material on the substrate; a buffer layer on a sideof the multi-layer stack of the absorber material opposite thesubstrate; and a transparent front contact on a side of the buffer layeropposite the multi-layer stack of the absorber material.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram illustrating an exemplary startingstructure for a hybrid vapor phase-solution phase growth process forCZTS(S,Se) including a first (vapor phase) CZTS(S,Se) layer formed on asubstrate according to an embodiment of the present invention;

FIG. 2 is a cross-sectional diagram illustrating a second (solutionphase) CZTS(S,Se) layer having been formed on a side of the first (vaporphase) CZTS(S,Se) layer opposite the substrate according to anembodiment of the present invention;

FIG. 3A is a cross-sectional diagram illustrating (optionally) formingone or more additional CZTS(S,Se) layers—in this example an additionalvapor phase CZTS(S,Se) layer—and how a concentration profile of theresulting CZTS(S,Se) stack can be tailored based on a composition of thelayers according to an embodiment of the present invention;

FIG. 3B is a cross-sectional diagram illustrating (optionally) formingone or more additional CZTS(S,Se) layers—in this example an additionalsolution phase CZTS(S,Se) layer—and how a concentration profile of theresulting CZTS(S,Se) stack can be tailored based on a composition of thelayers according to an embodiment of the present invention;

FIG. 4 is a diagram illustrating an exemplary methodology for forming aphotovoltaic device using the present hybrid vapor phase-solution phasegrowth process for CZTS(S,Se) according to an embodiment of the presentinvention;

FIG. 5A is a cross-sectional diagram illustrating an exemplaryphotovoltaic device which may be formed, for example, using themethodology of FIG. 4 having a two-layered CZTS(S,Se) stack according toan embodiment of the present invention;

FIG. 5B is a cross-sectional diagram illustrating an exemplaryphotovoltaic device which may be formed, for example, using themethodology of FIG. 4 having a three-layered CZTS(S,Se) stack accordingto an embodiment of the present invention;

FIG. 5C is a cross-sectional diagram illustrating another exemplaryphotovoltaic device which may be formed, for example, using themethodology of FIG. 4 having a three-layered CZTS(S,Se) stack accordingto an embodiment of the present invention;

FIG. 6A is a cross-sectional diagram illustrating use of an optionalsodium (Na) precursor layer between the substrate and the vapor phaseCZTS(S,Se) layer for doping the CZTS(S,Se) layer according to anembodiment of the present invention;

FIG. 6B is a cross-sectional diagram illustrating an alternativeembodiment wherein the optional Na precursor layer is placed on top ofthe vapor phase CZTS(S,Se) layer according to an embodiment of thepresent invention; and

FIG. 7 is a scanning electron micrograph (SEM) image of a two-layeredstack of CZT(S,Se) formed using the present hybrid vapor phase-solutionphase CZT(S,Se) techniques according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As provided above, while CZT(S,Se)-based photovoltaic devices have beenproduced with efficiencies at or above 12% using solution phasetechniques, defects in the resulting film such as pinholes can lead toshorting of the device when conductive contacts are added. Vapordeposition can be used to produce pinhole-free CZT(S,Se) films. However,the resulting devices have efficiencies well below that achieved withsolution phase processing.

As also provided above, it may be desirable to employ electron blockinglayers or hole transport layers for improved rectifying behavior andopen circuit voltage (Voc). However, these hole transport materials areoften formed from oxides such as tungsten trioxide (WO₃), nickel oxide(NiO), or molybdenum trioxide (MoO₃). All of these materials are easily(undesirably) reduced by hydrazine which is commonly employed insolution phase deposition.

Advantageously, provided herein are hybrid vapor phase-solution phaseCZT(S,Se) deposition techniques that remedy the above-described problemsthereby permitting the fabrication of high-efficiency CZT(S,Se)-basedphotovoltaic devices. Namely, as will be described in detail below, thepresent hybrid approach first leverages the benefits of a vapor phaseCZT(S,Se) process to produce a pinhole-free film. A vapor phase processdoes not involve solvents such as hydrazine, thus avoiding anyundesirable effects on underlying oxide materials.

The vapor phase CZT(S,Se) further acts as a barrier layer to protect anyunderlying oxide layers during a subsequent solution phase CZT(S,Se)process, thus preserving the functionality of the oxide layers. Thus, avariety of materials (including oxides such as indium-tin-oxide (ITO),fluorinated tin oxide (FTO), etc.—see below) can be used as substratesfor device fabrication in accordance with the present techniques. Thiswould not be possible with a purely solution based CZT(S,Se) approachinvolving hydrazine.

Additionally, the morphology at the interface between CZT(S,Se) and theback contact (e.g., molybdenum (Mo)) can be especially important in thindevices as more carriers are generated in this region. Vapor depositedCZT(S,Se) is void-free at this interface. Thus, according to the presenthybrid deposition approach, a void-free interface can be formed at theback contact.

The present hybrid approach next leverages the benefits of a solutionphase CZTS(S,Se) process to produce a high-efficiency film. As providedabove, solution deposition techniques for CZTS(S,Se) can produce deviceswith efficiencies at (or exceeding) 12%.

The term “CZT(S,Se),” as used herein, refers to a kesterite materialcontaining copper (Cu), zinc (Zn), tin (Sn), and at least one of sulfur(S) and selenium (Se). Thus, the designation (S,Se) indicates that Sand/or Se is present in the material.

A general description of the present hybrid vapor phase-solution phasegrowth process for CZT(S,Se) is now provided. An exemplaryimplementation of the present techniques in the fabrication of aphotovoltaic device will be provided below.

Referring to FIG. 1, the starting platform for the process is asubstrate 102. A variety of different substrates and substrateconfigurations are possible in accordance with the present techniques.It is to be understood that while FIG. 1 generically depicts substrate102 as a single layer, the substrate may in fact be formed from (e.g., astack of) multiple layers. By way of example only, substrate 102 mayinclude a conductive layer(s) coated on a transparent substrate.Conductive transparent substrate configurations are suited forphotovoltaic device applications. For instance, according to anexemplary embodiment, substrate 102 is Mo-coated (soda lime) glass.According to another exemplary embodiment, substrate 102 is zinc oxide(ZnO), zinc oxysulfate (ZnOS) and/or zinc tin oxide (ZnSnO)-coated ITOor FTO.

As noted above, with conventional processes that solely use solutionphase CZT(S,Se) deposition techniques with hydrazine as the solvent,degradation of oxide materials is inevitable. Advantageously, with thepresent techniques, the vapor phase CZT(S,Se) will act as a barrierlayer to protect the underlying oxide material(s) during subsequentsteps—thus making a greater variety of different substrateconfigurations possible.

Next as shown in FIG. 1, a CZT(S,Se) layer 104 is formed on thesubstrate 102 using vapor phase deposition. As will be described indetail below, the CZT(S,Se) will, according to an exemplary embodiment,be formed as a multi (e.g., two, three, etc.)-layered stack wherein afirst one of the CZT(S,Se) layers (e.g., CZT(S,Se) layer 104) is formedusing a vapor deposition techniques and a second one of the CZT(S,Se)layers is formed on the first CZT(S,Se) layer using a solution phasedeposition technique. Thus, the first layer formed on the substrate(e.g., CZT(S,Se) layer 104) may also be referred to herein as the firstCZT(S,Se) material/layer and the second layer formed on the first layermay also be referred to herein as the second CZT(S,Se) material/layer.Further, embodiments are also presented herein where one or moreadditional CZT(S,Se) layers are formed on the stack. Accordingly, theseadditional CZT(S,Se) layers may also be referred to herein as the third,fourth, etc. CZT(S,Se) materials/layers.

According to an exemplary embodiment, the vapor phase deposition ofCZT(S,Se) layer 104 includes contacting the substrate with a source ofCu, a source of Zn, a source of Sn, and at least one of a source of Sand a source of Se under conditions (e.g., temperature, pressure,duration, etc.) sufficient to form CZT(S,Se) layer 104 on the substrate102. According to an exemplary embodiment, the conditions include atemperature of from about 400 degrees Celsius (° C.) to about 450° C.,and ranges therebetween, and a pressure of from about 1×10⁻¹¹ torr toabout 1×10⁻³ torr, and ranges therebetween.

By way of example only, the formation of CZT(S,Se) layer 104 may becarried out in a vapor deposition system contained within a vacuumchamber wherein, for instance, the substrate 102 is placed on a hotplate within the vacuum chamber and heated to a temperature of fromabout 400° C. to about 450° C., and ranges therebetween. The sourcematerials for CZT(S,Se) layer 104 might simply be placed in crucibleswithin the vacuum chamber. However, in order to precisely control thefluxes of the Cu, Zn, Sn, S and/or Se, elemental sources of each thesecomponents are preferably contained in separate effusion cells connectedto the vacuum chamber that can be independently heated to control theflux. Suitable effusion cells for use in accordance with the presenttechniques are described, for example, in U.S. Pat. No. 6,162,300 issuedto Bichrt, entitled “Effusion Cell” (hereinafter “U.S. Pat. No.6,162,300”), the contents of which are incorporated by reference as iffully set forth herein. As described in U.S. Pat. No. 6,162,300, theflux from the effusion cells can be controlled by varying thetemperature of the effusion cell and/or by employing a control valvebetween the effusion cell and the growth chamber.

According to an exemplary embodiment, the CZT(S,Se) layer 104 isdeposited on the substrate 102 to a thickness of from about 50nanometers (nm) to about 300 nm, and ranges therebetween, e.g., fromabout 100 nm to about 300 nm, and ranges therebetween. By way of exampleonly, the above-described vapor phase deposition process can be carriedout for a duration sufficient to achieve a continuous layer having athickness in this range, wherein the duration can be anywhere from about5 minutes to about 300 minutes, and ranges therebetween.

Following deposition, CZT(S,Se) layer 104 is then subjected to a hightemperature anneal to generate a polycrystalline material with largegrain sizes. According to an exemplary embodiment, this annealing tofully crystallize the CZT(S,Se) layer 104 is performed at a temperatureof greater than about 500° C., e.g., from about 550° C. to about 650°C., and ranges therebetween. This will yield a CZT(S,Se) material havinggrains with an average grain size of from about 1 micrometer (μm) toabout 2 μm, and ranges therebetween. By way of example only, grain sizeis measured herein as the longest length dimension of the grain incross-section. As will be described in detail below, fully annealing theCZT(S,Se) layer 104 in this manner is needed to prevent damaging theCZT(S,Se) layer 104 during the solution deposition phase of the processsince the solvents used in the solutions can dissolve non-crystallineforms of the material. Full crystallization refers to the maximum grainsize achievable in the thin film. Vertically, grain size is limited tothe film thickness (the grains in the vertical dimension will grow nolarger than the film thickness). Laterally, however, grain size isdictated by the kinetics of grain boundary formation. When fullycrystallized, each grain is crystalline, i.e., X-ray diffraction showscharacteristic lines whose widths are representative of the size of thegrains. Once grains achieve these dimensions (e.g., a grain size of fromabout 1 μm to about 2 μm, and ranges therebetween—measured as thelongest length dimension of the grain in cross-section), no furthercoarsening is observed (the grains do not get larger) and so the film isthen fully crystallized.

An advantage of a vapor phase deposition process is that the resultingCZT(S,Se) is pinhole-free. As noted above, pinholes are an inevitableresult of solution phase CZT(S,Se) deposition and can undesirably leadto shorting when contacts are formed on the device (especially in thecase of devices having thin absorber layers). By employing the presenttwo-layered stack configuration, the vapor phase CZT(S,Se) layer 104first formed in the stack is thus pinhole-free.

As also provided above, the vapor phase CZT(S,Se) acts as a barrierlayer to protect underlying oxide materials during subsequent processingsteps. Specifically, forming the second layer in the two-layered stackwill involve a solution phase deposition process that might include theuse of hydrazine which can undesirably reduce the underlying oxidematerials. Thus, as shown in FIG. 1, it is desirable to form the (vaporphase) CZT(S,Se) layer 104 as a continuous layer that fully covers andprotects the substrate 102 (i.e., so as to protect oxide materials, ifany, present in the substrate).

Optionally, the CZT(S,Se) layer 104 is doped with sodium (Na). Na dopingcan serve to both coarsen/enlarge the size of the individual crystalgrains, and assist in passivating the grain boundaries so as to increasedevice efficiency. See, for example, Nagaoka et al., “Effects of sodiumon electrical properties in Cu₂ZnSnS₄ single crystal,” Applied PhysicsLetters 104, 152101 (April 2014) (hereinafter “Nagaoka”), the contentsof which are incorporated by reference as if fully set forth herein.Nagaoka describes the effects of Na diffusion on CZTS grain size.Further, according to Nagaoka, there is an enhancement in the unit-cellsize of the CZTS with increases in the Na concentration—based on Nabeing a substitutional impurity in CZTS since the radius of the Na atomis greater than that of cations in CZTS. Also, the hole mobility of theCZTS is enhanced with increasing Na concentration.

According to an exemplary embodiment, Na doping of the CZT(S,Se) layer104 is performed by placing a Na precursor layer on the bottom (see FIG.6A) or on the top (see FIG. 6B) of the CZT(S,Se) layer 104. When thehigh temperature crystallizing anneal of CZT(S,Se) layer 104 isperformed, Na ions from the precursor layer diffuse into and dope theCZT(S,Se) layer 104. According to an exemplary embodiment, the Naprecursor layer is a layer of sodium fluoride (NaF). Referring brieflyto FIG. 6A, this figure illustrates the optional step of placing a Naprecursor layer 602 a between the substrate 102 and the CZTS(S,Se) layer104. According to an exemplary embodiment, the Na precursor layer 602 a(e.g., a NaF layer) is first deposited onto the substrate 102 using adeposition process such as chemical vapor deposition (CVD). Thesubstrate 102 with the Na precursor layer 602 a formed thereon can thenbe transferred to the vapor deposition system and the CZTS(S,Se) layer104 can be formed on a side of the Na precursor layer 602 a opposite thesubstrate using the above-described process.

Na ions from the Na precursor layer 602 a diffuse into the CZTS(S,Se)layer 104 during the high temperature anneal (used to generate apolycrystalline material with large grain sizes—see above). Theamount/concentration of the Na dopant into the CZTS(S,Se) layer 104 canbe controlled, for example, based on the thickness of the Na precursorlayer 602 a—with a thicker Na precursor layer 602 a contributing to agreater amount of the Na dopant, and vice versa. One skilled in the art,given the instant description of the present techniques, would be ableto tailor the thickness of the Na precursor layer 602 a for a desired Nadopant concentration. Once the Na ion diffusion via the high temperatureanneal has taken place, the Na precursor layer is no longer present.

Alternatively, FIG. 6B illustrates the optional step of placing a Naprecursor layer 602 b on top of the CZTS(S,Se) layer 104. As shown inFIG. 6B, once the CZTS(S,Se) layer 104 is formed on the substrate 102 asdescribed above, the Na precursor layer 602 b (e.g., a NaF layer) canthen be formed (e.g., via CVD) on a side of the CZTS(S,Se) layer 104opposite the substrate 102.

Na ions from the Na precursor layer 602 b diffuse into the CZTS(S,Se)layer 104 during the high temperature anneal (see above). Theamount/concentration of the Na dopant into the CZTS(S,Se) layer 104 canbe controlled, for example, based on the thickness of the Na precursorlayer 602 b—with a thicker Na precursor layer 602 b contributing to agreater amount of the Na dopant, and vice versa. One skilled in the art,given the instant description of the present techniques, would be ableto tailor the thickness of the Na precursor layer 602 b for a desired Nadopant concentration. Once the Na ion diffusion via the high temperatureanneal has taken place, the Na precursor layer is no longer present.

Referring back now to FIG. 2, the next phase in the present hybridapproach is to form a solution phase CZTS(S,Se) layer on the (vaporphase) CZTS(S,Se) layer 104. Specifically, as shown in FIG. 2, a(solution phase) CZTS(S,Se) layer 202 is formed on a side of theCZTS(S,Se) layer 104 opposite the substrate 102. As provided above,CZTS(S,Se) layer 104 may also be referred to herein as a firstCZTS(S,Se) material/layer, and CZTS(S,Se) layer 202 may also be referredto herein as a second CZTS(S,Se) material/layer.

Generally, CZTS(S,Se) layer 202 is formed on CZTS(S,Se) layer 104 usinga solution phase deposition process. According to an exemplaryembodiment, the device structure (i.e., the substrate 102 with theCZTS(S,Se) layer 104 formed thereon) is removed from the vapordeposition system and placed in a nitrogen glove box. Use of a nitrogenglove box avoids exposure to oxygen and water vapor.

A casting process (such as spin-coating) is then used to deposit one ormore CZTS(S,Se) precursor solutions (e.g., inks, slurries, etc.) to formCZTS(S,Se) layer 202 on CZTS(S,Se) layer 104. By way of example only,suitable inks for kesterite-type Cu—Zn—Sn—(Se,S) materials are providedin U.S. Patent Application Publication Number 2011/0094557 by Mitzi etal., entitled “Method of Forming Semiconductor Film and PhotovoltaicDevice Including the Film” (hereinafter “U.S. Patent ApplicationPublication Number 2011/0094557”), the contents of which areincorporated by reference as if fully set forth herein. A suitableprocess for preparing and depositing (e.g., using one or morespin-coating steps with intermediate drying steps to achieve adequatecoverage) a CZTSe or CZTSSe slurry are described, for example, in U.S.Pat. No. 8,765,518 issued to Teodor K. Todorov, entitled “Chalcogenidesolutions,” the contents of which are incorporated by reference as iffully set forth herein.

Advantageously, the resulting hybrid vapor phase-solution phaseCZT(S,Se) has been found to be pinhole-free. Specifically, ashighlighted above, when trying to deposit thin CZT(S,Se) films solelyfrom the solution phase one is left with pinholes that can short orshunt the device. However, in the present hybrid device no internalpinholes are present. Thus the present hybrid process resolves thepinhole problem.

It is important to have the vapor-deposited CZTS(S,Se) layer 104 fullyannealed (prior to the solution deposition phase) since partiallyannealed CZTS(S,Se) material may dissolve as the solution(s) (ink,slurry, etc.) is deposited thereby rendering the device inoperative. Ashighlighted above, by “fully annealed” it is meant that the material isfully crystallized. Further, while a solution phase deposition processfor CZTS(S,Se) yields high efficiency devices, some notable drawbackshowever include pinhole formation in the resulting material and that thesolvents used for solution deposition (such as hydrazine) undesirablydegrade oxide materials. For instance, the inks provided in U.S. PatentApplication Publication Number 2011/0094557 contain hydrazine. Any oxidematerial(s) exposed during the deposition of the inks would undesirablybe reduced by the hydrazine rendering them ineffective. Advantageously,the vapor phase CZTS(S,Se) layer 104 acts as a barrier layer protectingany underlying oxide materials during the solution deposition phase.

According to an exemplary embodiment, the present techniques areemployed to form thin devices wherein CZTS(S,Se) layer 104 andCZTS(S,Se) layer 202 combined have a sub-micrometer thickness (i.e.,less than 1 micrometer). Specifically, the combined thickness ofCZTS(S,Se) 104 and CZTS(S,Se) layer 202 in this example is preferablyfrom about 100 nm to about 800 nm, and ranges therebetween. Accordingly,since CZTS(S,Se) layer 104 can have a thickness of from about 50 nm toabout 300 nm, and ranges therebetween, e.g., from about 100 nm to about300 nm, then CZTS(S,Se) layer 202 would, in that case, preferably have athickness of from about 50 nm to about 500 nm, and ranges therebetween,e.g., from about 100 nm to about 500 nm, and ranges therebetween. Asprovided above, contact engineering (using high work function backcontacts) in conjunction with a sub-micrometer CZTS(S,Se) thick absorbercan be used to increase Voc.

In accordance with the present techniques, the CZTS(S,Se) layer 104serves as a seed layer for CZTS(S,Se) layer 202. Specifically, followingdeposition of CZTS(S,Se) layer 202 on CZTS(S,Se) layer 104, thetwo-layered stack is annealed to fully crystallize the CZTS(S,Se) layer202. According to an exemplary embodiment, the anneal is performed at atemperature of greater than about 500° C., e.g., from about 550° C. toabout 650° C., and ranges therebetween. Following this anneal, there areno structural barriers between CZTS(S,Se) layer 104 and CZTS(S,Se) layer202. For instance, in cross-sectional images of the hybrid material onecannot see any hint of an “interface” between the vapor phase andsolution phase layers. See, for example, FIG. 7—described below.

The stack may be complete with the two layers, i.e., CZTS(S,Se) layer104 and CZTS(S,Se) layer 202. Optionally, however, one or moreadditional vapor phase and/or solution phase CZTS(S,Se) layers may beformed on the stack if so desired. Adding additional layers to the stackmay be done, for example, in order to control the concentration profileof the material. For instance, regulating the S to Se ratio inCZTS(S,Se) absorber materials is important in terms of band gap tuningand/or band gap grading. See, for example, U.S. patent Ser. No.14/499,116 by Mahajan et al., entitled “Anneal Techniques forChalcogenide Semiconductors,” the contents of which are incorporated byreference as if fully set forth herein. Thus, by way of example only, athree-layer stack can be formed consisting of vapor phase/solutionphase/vapor phase CZTS(S,Se) layers wherein the S to Se ratio is variedbetween the vapor phase and the solution phase layers to achieve anoscillatory concentration profile throughout the stack. For example, thevapor phase layer(s) in the (two-, three-, etc.) layer stack can beconfigured to be either a full S CZTS(S,Se) layer (i.e., no Se—a.k.a.“Se-free”) or a full Se CZTS(S,Se) layer (i.e., no S—a.k.a. “S-free”),while the solution phase layer(s) in the (two-, three-, etc.) layerstack can be configured to have both S and Se—or vice versa—therebyforming an oscillatory (S,Se) concentration profile throughout the stack(see FIG. 3A). Alternatively, one of the vapor phase or solution phaselayers in the (two-, three-, etc.) layer stack can be configured to haveboth S and Se while another vapor phase or solution phase layer(s) inthe (two-, three-, etc.) layer stack is configured to be either a full SCZTS(S,Se) layer (i.e., no Se—a.k.a. “Se-free”) or a full Se CZTS(S,Se)layer (i.e., no S—a.k.a. “S-free”)—or vice versa—thereby also forming anoscillatory (S,Se) concentration profile throughout the stack (see FIG.3B). It is notable that the process described above would be implementedin the same manner to form one or more additional layers on the stack.

For illustrative purposes only, FIG. 3A depicts an exemplaryconfiguration wherein a third CZTS(S,Se) layer—a vapor phase CZTS(S,Se)layer 302 a—is formed on the stack, and FIG. 3B depicts an exemplaryconfiguration wherein a third CZTS(S,Se) layer—a solution phaseCZTS(S,Se) layer 302 b—is formed on the stack. The result in either caseis a three-layer CZTS(S,Se) stack. CZTS(S,Se) layer 302 a or CZTS(S,Se)layer 302 b is formed on a side of CZTS(S,Se) layer 202 oppositeCZTS(S,Se) layer 104 in the same manner as described above for the vaporor solution phase deposition of CZTS(S,Se) layer 104 or CZTS(S,Se) layer202, respectively. FIGS. 3A and 3B also illustrate how individuallytuning the composition of the layers can be used to control theconcentration profile of the CZTS(S,Se) material. For instance, in theexamples shown in FIGS. 3A and 3B, the S to Se ratio is variedthroughout the layers—such that an oscillatory (S,Se) concentrationprofile is achieved throughout the stack. Of course this is only anexample, and one skilled in the art given the present teachings couldtailor the concentration profile for a variety of differentapplications. Once formation of the CZTS(S,Se) stack is completed, theremaining layers/structures can be added to form a functioning device.

An exemplary implementation of the present techniques in the fabricationof a photovoltaic device is now provided by way of reference tomethodology 400 of FIG. 4. For clarity, a summary of the steps includedin the present techniques is provided in FIG. 4. In step 402, asubstrate is provided. As described above, a variety of differentsubstrate configurations are possible—including either single ormulti-layered substrates. Advantageously, in accordance with the presenttechniques, the substrate can include oxide materials since the firstCZTS(S,Se) layer will be deposited using a vapor phase process and willserve as a barrier layer to protect any underlying oxide materialsduring subsequent solution phase processes.

Optionally, in step 404 a Na precursor layer (e.g., a NaF layer) is usedto dope the first CZTS(S,Se) layer (see below). Namely, when a hightemperature crystallizing anneal of the first CZT(S,Se) layer isperformed, Na ions from the precursor layer diffuse into and dope thefirst CZT(S,Se) layer. Alternatively, the Na precursor layer can beformed on top of the first CZT(S,Se) layer (see optional step 408,described below).

In step 406, the first CZTS(S,Se) layer is deposited on the substrate(or on a side of the Na precursor layer opposite the substrate, ifpresent) using vapor phase deposition. As provided above, this vapordeposition step includes contacting the substrate with a source of Cu, asource of Zn, a source of Sn, and at least one of a source of S and asource of Se under conditions (e.g., temperature, pressure, duration,etc.) sufficient to form the first CZT(S,Se) layer on the substrate.According to an exemplary embodiment, the conditions include atemperature of from about 400° C. to about 450° C., and rangestherebetween, and a pressure of from about 1×10⁻¹¹ torr to about 1×10-3torr, and ranges therebetween. The vapor deposition is preferablycarried out in this step for a duration that results in the firstCZT(S,Se) layer being formed to a thickness of from about 50 nm to about300 nm, and ranges therebetween, e.g., from about 100 nm to about 300nm, and ranges therebetween, on the substrate.

An exemplary vacuum chamber-based vapor deposition system was describedin detail above wherein the Cu, Zn, Sn, S and/or Se source materials arecontained in separate effusion cells that can be each independentlyheated to control the flux. That description is incorporated byreference herein.

If Na doping is desired, and the (optional) Na precursor layer was notformed between the substrate and the first CZT(S,Se) layer in step 404,then in step 408 the (optional) Na precursor layer can alternatively beformed on top of the first CZT(S,Se) layer (i.e., on a side of the firstCZT(S,Se) layer opposite the substrate). As provided above, when thehigh temperature crystallizing anneal of the first CZT(S,Se) layer isperformed, Na ions from the precursor layer diffuse into and dope thefirst CZT(S,Se) layer.

Specifically, in step 410 the first CZT(S,Se) layer is then fullyannealed, i.e., to fully crystallize the first CZT(S,Se) layer.According to an exemplary embodiment, the annealing in step 410 isperformed at a temperature of greater than about 500° C., e.g., fromabout 550° C. to about 650° C., and ranges therebetween. This will yielda first CZT(S,Se) layer having grains with an average grain size of fromabout 1 μm to about 2 μm, and ranges therebetween—see above. Asdescribed in detail above, fully crystallizing the first CZT(S,Se) layeris important since the subsequent solution phase CZT(S,Se) depositioncan undesirably dissolve any non-crystal material. If the (optional) Naprecursor layer is employed (as per either step 404 or step 408, above)then it is during this high temperature crystallization anneal in step410 that the Na ions from the precursor layer will diffuse into and dopethe first CZT(S,Se) layer. Once this diffusion has taken place, the Naprecursor layer is no longer present. Thus, in the next step in theprocess a second CZT(S,Se) layer is formed on the (in this case Nadoped) first CZT(S,Se) layer.

Specifically, in step 412, the second CZT(S,Se) layer is formed on aside of the first CZT(S,Se) layer opposite the substrate using solutionphase deposition. As provided above, this solution deposition stepincludes using a casting process (such as spin-coating) to deposit oneor more CZTS(S,Se) precursor solutions (e.g., inks, slurries, etc.) toform the CZTS(S,Se) layer on the first CZTS(S,Se) layer. As described indetail above, the fully crystallized first CZTS(S,Se) layer acts as abarrier layer to protect any underlying oxide materials that aresensitive to solvents (such as hydrazine) used in the precursorsolutions and/or casting process.

According to an exemplary embodiment, the combined thickness of thefirst CZTS(S,Se) layer and the second CZTS(S,Se) layer is from about 100nm to about 800 nm, and ranges therebetween. Accordingly, since thefirst CZTS(S,Se) layer can have a thickness of from about 50 nm to about300 nm, and ranges therebetween, e.g., from about 100 nm to about 300 nm(see above), then the second CZTS(S,Se) layer would, in that case, havea thickness of from about 50 nm to about 500 nm, and rangestherebetween, e.g., from about 100 nm to about 500 nm, and rangestherebetween. As provided above, thin devices (i.e., devices having anabsorber with a thickness of less than 1 m) enable the use of contactengineering (with high work function back contacts) to increase Voc.

The result of forming the second CZT(S,Se) layer on the first CZT(S,Se)layer is a two-layered CZT(S,Se) stack. As described in detail above(and below), one or more additional CZT(S,Se) layers (vapor phase and/orsolution phase) may be formed on the stack if so desired. A motivatingfactor for forming additional CZT(S,Se) layers is that doing so enablesfurther fine-tuning of the concentration profile of the CZT(S,Se)material, for example, to achieve an oscillatory concentration profilethroughout the stack. Of course, if a goal is to keep the overallthickness of the CZTS(S,Se) stack less than 1 μm (e.g., from about 100nm to about 800 nm, and ranges therebetween), then care must be taken totailor the thickness of each of the individual layers in the stack tomeet this requirement.

In step 414, the two-layered stack is fully annealed to fullycrystallize the second CZTS(S,Se) layer. According to an exemplaryembodiment, this anneal in step 414 is performed at a temperature ofgreater than about 500° C., e.g., from about 550° C. to about 650° C.,and ranges therebetween. Following this anneal, there are no structuralbarriers between the first and second CZTS(S,Se) layers. Further, asprovided above, the already-fully crystallized first CZTS(S,Se) layeracts as a seed layer for the crystallization of the second CZTS(S,Se)layer. Even if additional vapor phase and/or solution phase CZTS(S,Se)layers are to be deposited onto the stack, it is preferable to performthis anneal in step 414 prior to depositing the additional CZTS(S,Se)layers. For instance, the anneal performed in step 414 serves to bakeoff the remaining solvent from the solution phase deposition—withoutwhich further vapor phase deposition steps could not be performed.

Optionally, in step 416 at least one more additional (vapor phase and/orsolution phase) CZTS(S,Se) layers are formed on the stack. An exampleillustrating an additional vapor phase CZTS(S,Se) layer having beendeposited onto the stack is shown in FIG. 3A—described above, and anexample illustrating an additional solution phase CZTS(S,Se) layerhaving been deposited onto the stack is shown in FIG. 3B—describedabove. However, any number and/or combination of additional vapor phaseand/or solution phase CZTS(S,Se) layers may be formed on the stack usingthe teachings provided herein. Preferably, after the deposition of eachadditional layer on the stack an anneal is performed as per step 414—seeFIG. 4. As provided above, this insures compatibility between the layersin the stack.

Once the CZT(S,Se) stack formation is complete, any additionalprocessing steps may be performed to form a completed device. By way ofexample only, the remaining steps of methodology 400 are directed to anexemplary embodiment for forming a photovoltaic device. It is to beunderstood, however, that the present techniques are more broadlyapplicable to any application involving a CZTS(S,Se) material for whichthe present hybrid vapor phase-solution phase CZT(S,Se) techniques maybe employed. In this example, the CZT(S,Se) layers in the stackcollectively serve as an absorber material of the photovoltaic device.

In step 418, for example, a buffer layer is formed on a side of theCZT(S,Se) stack opposite the substrate. According to an exemplaryembodiment, the buffer layer is formed from at least one of cadmiumsulfide (CdS), a cadmium-zinc-sulfur material of the formulaCd_(1-x)Zn_(x)S (wherein 0<x≤1), indium sulfide (In₂S₃), zinc oxide,zinc oxysulfide (e.g., a Zn(O,S) or Zn(O,S,OH) material), and aluminumoxide (Al₂O₃). The buffer layer may be formed on the CZT(S,Se) stackusing a deposition process such as standard chemical bath deposition.According to an exemplary embodiment, the buffer layer is formed havinga thickness of from about 50 angstroms (Å) to about 1,000 Å, and rangestherebetween. The buffer layer and the CZT(S,Se) absorber material forma p-n junction therebetween.

In step 420, a transparent front contact is formed on a side of thebuffer layer opposite the CZT(S,Se) stack. According to an exemplaryembodiment, the transparent front contact is formed from a transparentconductive oxide (TCO) such as indium-tin-oxide (ITO) and/or aluminum(Al)-doped zinc oxide (ZnO) (AZO)). The transparent front contact may beformed on the buffer layer using a deposition process such assputtering. At this point in the process, a functioning photovoltaicdevice has been formed. However, additional processing may be performedand/or additional structures may be formed if so desired.

For example, in step 422 metal contacts may optionally be formed on aside of the transparent front contact opposite the buffer layer.According to an exemplary embodiment, the metal contacts are formed fromaluminum (Al) and/or nickel (Ni). The metal contacts may be formed onthe transparent front contact using a process such as electron-beamevaporation.

Another optional component of the device is an antireflective coating.For example, in step 424 an antireflective coating may be formed on aside of the transparent front contact opposite the buffer layer (so asto cover the metal contacts, if present). According to an exemplaryembodiment, the antireflective coating is formed from magnesium oxide(MgO) or magnesium fluoride (MgF₂). The antireflective coating may beformed on the transparent front contact using a deposition process suchas electron-beam evaporation.

FIG. 5A is a diagram illustrating an exemplary photovoltaic device 500Awhich may be formed, for example, using methodology 400 of FIG. 4. Thesame reference numerals (and patterning) are being used to denote to thesame structures throughout the figures. As shown in FIG. 5A,photovoltaic device 500A includes a substrate 102, a multi-layer stack504 of CZT(S,Se) absorber material on the substrate 102, a buffer layer506 on a side of the multi-layer stack 504 of CZT(S,Se) absorbermaterial opposite the substrate 102, a transparent front contact 508 ona side of the buffer layer 506 opposite the multi-layer stack 504 ofCZT(S,Se) absorber material, metal contacts 510 on a side of thetransparent front contact 508 opposite the buffer layer 506, and anantireflective coating 512 on a side of the transparent front contact508 opposite the buffer layer 506 (so as to cover the metal contacts510).

As provided above, a variety of different substrate configurations maybe used in accordance with the present techniques—including substratescontaining oxide materials. Namely, the vapor phase CZT(S,Se) layerformed on the substrate acts as a barrier layer to protect anyunderlying oxide materials from solvents (such as hydrazine) used duringsubsequent solution phase deposition steps. To illustrate this point,according to an exemplary embodiment, substrate 102 can include amultilayer oxide configuration wherein, for example, substrate 102includes a transparent conductive oxide (TCO) 501, such as ITO or FTO,coated with an oxide layer 502 containing ZnO, ZnOS and/or ZnSnO.Alternatively, substrate 102 can include soda lime glass 501 coated witha conductive layer 502 of Mo.

In the exemplary embodiment shown in FIG. 5A, the multi-layer stack 504of CZT(S,Se) absorber material is a two-layered stack including a first(vapor phase) CZT(S,Se) layer 104 and a second (solution phase)CZT(S,Se) layer 202. However, as provided above, embodiments areanticipated herein where at least one additional vapor phase and/orsolution phase CZT(S,Se) layer is also present in the stack. See, forexample, FIGS. 5B and 5C, below.

As provided above, the buffer layer 506 may be formed from at least oneof CdS, a cadmium-zinc-sulfur material of the formula Cd_(1-x)Zn_(x)S(wherein 0<x≤1), In₂S₃, zinc oxide, zinc oxysulfide (e.g., a Zn(O,S) orZn(O,S,OH) material), and Al₂O₃. The transparent front contact 508 maybe formed from a TCO such as ITO and/or Al-doped ZnO (AZO). The metalcontacts 510 may be formed from Al and/or Ni. The antireflective coating512 may be formed from MgO or MgF₂.

For completeness, FIGS. 5B and 5C are provided which illustrate the samephotovoltaic device structure as FIG. 5A except that an additional vaporor solution phase CZT(S,Se) layer is present in (this case athree-layered) stack. Specifically, in the photovoltaic device 500Bshown in FIG. 5B, the multi-layer stack 504 of CZT(S,Se) absorbermaterial is a three-layered stack including a first (vapor phase)CZT(S,Se) layer 104, a second (solution phase) CZT(S,Se) layer 202, anda third (vapor phase) CZT(S,Se) layer 302 a. In the photovoltaic device500C shown in FIG. 5C, the multi-layer stack 504 of CZT(S,Se) absorbermaterial is a three-layered stack including a first (vapor phase)CZT(S,Se) layer 104, a second (solution phase) CZT(S,Se) layer 202, anda third (solution phase) CZT(S,Se) layer 302 b. As above, the samereference numerals (and patterning) are being used to denote to the samestructures throughout the figures.

FIG. 7 is a scanning electron micrograph (SEM) image 700 of atwo-layered stack of CZT(S,Se) formed using the present hybrid vaporphase-solution phase CZT(S,Se) techniques on a Mo-coated substrate. Itis notable from SEM image 700 that the hybrid deposited materialexhibits virtually no voiding at the Mo/CZT(S,Se) interface and showslarge single grains that span the width of the film. It is also notablethat there is no observable interface between the vapor and solutionphase grown layers.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. A photovoltaic device, comprising: a substrate; a layer of a first kesterite material on the substrate, wherein the first kesterite material comprises copper (Cu), zinc (Zn), tin (Sn), and at least one of sulfur (S) and selenium (Se); a layer of a second kesterite material on a side of the first kesterite material opposite the substrate, wherein the second kesterite material comprises Cu, Zn, Sn, and at least one of S and Se, and wherein the first kesterite material and the second kesterite material form a multi-layer stack of an absorber material on the substrate; a buffer layer on a side of the multi-layer stack of the absorber material opposite the substrate; and a transparent front contact on a side of the buffer layer opposite the multi-layer stack of the absorber material.
 2. The photovoltaic device of claim 1, further comprising: at least one layer of a third kesterite material on the multi-layer stack of the absorber material.
 3. The photovoltaic device of claim 2, wherein the third kesterite material comprises Cu, Zn, Sn, and at least one of S and Se.
 4. The photovoltaic device of claim 2, wherein the first kesterite material and the third kesterite material both comprise S and are both Se-free, and wherein the second kesterite material comprises both S and Se, such that an oscillatory concentration profile is present throughout the multi-layer stack of the absorber material.
 5. The photovoltaic device of claim 2, wherein the first kesterite material and the third kesterite material both comprise Se and are both S-free, and wherein the second kesterite material comprises both S and Se, such that an oscillatory concentration profile is present throughout the multi-layer stack of the absorber material.
 6. The photovoltaic device of claim 5, wherein an oscillatory concentration profile of S and Se is present throughout the multi-layer stack of the absorber material.
 7. The photovoltaic device of claim 1, wherein the layer of the first kesterite material has a thickness of from about 50 nm to about 300 nm, and ranges therebetween.
 8. The photovoltaic device of claim 1, wherein the layer of the first kesterite material has a thickness of from about 100 nm to about 300 nm, and ranges therebetween.
 9. The photovoltaic device of claim 1, wherein the layer of the first kesterite material is pinhole-free.
 10. The photovoltaic device of claim 1, wherein the first kesterite material has an average grain size of from about 1 μm to about 2 μm, and ranges therebetween.
 11. The photovoltaic device of claim 1, wherein the first kesterite material is doped with sodium (Na).
 12. The photovoltaic device of claim 1, wherein the layer of the second kesterite material has a thickness of from about 50 nm to about 500 nm, and ranges therebetween.
 13. The photovoltaic device of claim 1, wherein the layer of the second kesterite material has a thickness of from about 100 nm to about 500 nm, and ranges therebetween.
 14. The photovoltaic device of claim 1, wherein the substrate comprises a transparent conductive oxide (TCO) on which an oxide material is disposed.
 15. The photovoltaic device of claim 14, wherein the transparent conductive oxide (TCO) is selected from the group consisting of: indium tin oxide (ITO) and fluorinated tin oxide (FTO).
 16. The photovoltaic device of claim 1, further comprising: metal contacts on a side of the transparent front contact opposite the buffer layer.
 17. The photovoltaic device of claim 16, wherein the metal contacts comprise a material selected from the group consisting of: aluminum (Al), nickel (Ni), and combinations thereof.
 18. The photovoltaic device of claim 16, further comprising: an antireflective coating on the transparent front contact covering the metal contacts.
 19. The photovoltaic device of claim 18, wherein the antireflective coating comprises a material selected from the group consisting of: magnesium oxide (MgO) and magnesium fluoride (MgF₂).
 20. The photovoltaic device of claim 1, wherein the buffer layer comprises a material selected from the group consisting of: cadmium sulfide (CdS), a cadmium-zinc-sulfur material of the formula Cd_(1-x)Zn_(x)S (wherein 0<x≤1), indium sulfide (In₂S₃), zinc oxide, zinc oxysulfide, aluminum oxide (Al₂O₃), and combinations thereof. 